1. Field
Photolytically induced redox curable compositions, which show a delayed onset of cure resulting in controllable “open times” for the end user consumer, are provided. These compositions are activated by exposure to electromagnetic radiation, such as UV/VIS radiation with a wavelength in the range of 254-470 nm, but show a commercially meaningful “open time” prior to cure. This property is particularly attractive when opaque substrates and/or substrates with low transmissivity are to be assembled.
2. Brief Description of Related Technology
Photoinitiated curable adhesive compositions based on epoxy resins have many benefits and advantages desirable to end user consumers. For example, such compositions provide cured products having physical properties comparable to those from a thermosetting composition (though without the application of heat necessary to cure such compositions which oftentimes may compromise the integrity of the substrate to be joined or other components thereon).
Photoinitiated adhesive compositions have, therefore, been used in various commercial applications such as a liquid crystal display and lamination in a digital video disk.
Photoinitiated, cationically curable epoxy resin compositions, which contain a ferrocenium complex salt, a compound containing cycloaliphatic epoxide groups as well as a mixture of polyethylene glycol and Y-butyrolactone as a solvent, have been disclosed in European Patent Document No. EP 344 910. The composition so disclosed is reported by intended for use for protective coatings and as a binder for magnetic particles or abrasives and as binders and in the printing industry.
Photoinitiated, cationically curable epoxy resin compositions, which contain trivalent phenols, an organic peroxide, a ferrocenium complex salt and a cycloaliphatic epoxy, are also known. See e.g. German Patent Document No. DE 20 25 814.
It is known that cure rate is dependent upon catalyst concentration and light intensity. For instance, page 3965 of J. V. Crivello and J. L. Lee, J. Poly. Sci.: Part A, 27, 3951-68 (1989) presents a table (reproduced below) that shows the effect of photoinitiator concentration on the UV cure of the epoxy, limonene dioxide (a cycloaliphatic epoxy).
PhotoinitiatorUV Cure Rate*Concentration2 Lamps1 Lamp1 Lamp1 Lamp(%)(300 W)(300 W)(200 W)(120 W)0.5>500350—501.0>500>500—1002.0>500>500>5001503.0>500>500>500250*Determined on a RPC UV Processor using 1 mil films on glass substrates; cure rates are in ft/min.
However, when these findings are used in practice to attempt to produce a delay in gellation (or increase in open time) in epoxy compositions, the relatively low level of active catalyst species becomes depleted by quenching reactions with trace contaminants and/or ambient moisture. This effect has been recognized by M. D. Soucek and J. Chen, J. of Coatings Tech., 75, 937 (February 2003). See also FIG. 3 herein, which illustrates the lower level of percent cure.
International Patent Application No. PCT/US05/31567 is directed to and claims a photoinitiated cationically curable composition, comprising:
(a) an aliphatic epoxy resin, at least a portion of which includes an aliphatic glycidyl ether epoxy resin;
(b) a cationic photoinitiator; and
(c) a free radical initiator.
The use of epoxy-based compositions in this regard is oftentimes disfavored due in part to the yellowing that may be observed with such compositions and the heat of reaction generated by such compositions. The former can comprise aesthetics; the latter can comprise the integrity of the substrates being assembled.
U.S. Pat. No. 4,525,533 (Rooney) and U.S. Pat. No. 4,533,446 (Conway) are each directed to acrylate-based compositions. The '533 patent provides an acrylic monomer and a peroxy initiator, together with what is reported to be a ferrocenium salt. The '446 patent provides a photoactivatable anaerobic adhesive composition comprised of (a) an anaerobically curable acrylate ester monomer, (b) a compound of a certain structure which decomposes upon exposure to ultraviolet or visible light radiation to release a strong acid; (c) a peroxy free radical initiator capable of initiating the polymerization of the monomer in the substantial absence of oxygen, and (d) a ferrocenyl activator of anaerobic polymerization which in the presence of a strong acid reacts with the peroxide initiator to produce free radicals to catalyze the anaerobic polymerization of the monomer.
Despite the state of the technology, it would be desirable to provide end user consumers with a variety of alternative technologies with which to satisfy their product assembly needs, particularly when it comes to improving the open time of adhesives which gives end user consumers greater flexibility in their assembly procedures. In addition, it would be desirable to provide end user consumers with technologies that offer improved performance properties, such as shear strength.
To that end, there has been an on-going, long felt, but as of yet unmet, desire to find photoinitiated adhesive compositions, with an open time and a controllably delayed cure profile. Such a feature provides an open time and flexibility to the end user when designing processes for assemblying parts with such an adhesive composition.